The present invention relates to an apparatus for jitter generation used in measurement systems which evaluate the specifications of digital communication receivers as well as providing a general method of synthesis of programmable periods.
Stress testing of digital communication receivers is currently performed using a digital data source with the property that its output test waveform may be modulated in time in some predetermined way. This data source is known as a jitter generator. This source is composed of a radio-frequency (RF) synthesizer which is phase or frequency modulated by a second, lower frequency synthesizer to produce a sinusoidal clock signal with timing modulation (jitter). This sinusoidal signal is then amplitude limited to produce a digital square waveform. This output is then sent to a digital circuit which will produce the desired test sequence for the device under test. Such a jitter generator is used for the test of many standard communication links such as specified in The International Telegraph and Telephone Consultative Committee (hereafter referred to as CCITT) Recommendation "Timing Jitter Measuring Equipment for Digital Systems", Vol. IV, Rec. 0.171, which is incorporated by reference. An example of an apparatus used for jitter generation is described by P. R. Trischitta, and P. Sannuti, "The Jitter Tolerance of Fiber Optic Regenerators", IEEE Transactions on Communications, Vol. COM-35, NO. 12, December 1987, which is hereby incorporated by reference. In both of these references the "Jitter Generator" is formed using a lower frequency sine wave generator which modulates a high frequency RF synthesizer followed by a data generator.
The principal limitations of this approach are:
First, RF synthesizers are not easily modulated by amounts in excess of several tens of cycles of the output period due to the analog nature of the internal modulation circuits, thus the range of the jitter amplitude that can be used in a jitter tolerance measurement is limited. Second, the exact amount of movement in time of the test signal produced by the RF synthesizer is not known accurately and must be characterized, or calibrated, externally.
Third, the jitter bandwidth (modulation bandwidth) of these techniques is limited by the modulation capability of the synthesizer. The result is a modulation of only several hundreds kilohertz of sine wave modulation. Finally, this approach to jitter generation requires multiple pieces of measurement equipment.
It is desirable to provide a jitter generation capability with broader jitter bandwidth, sinusoidal and non-sinusoidal jitter generation, much larger jitter amplitude, a calibrated jitter amplitude, and a more compact system design in order to more accurately characterize high speed receiver components. However, the synthesized control of waveform period should be retained.